Automatic transmissions for motor vehicles commonly employ torque transmitting devices for the selective transmission of torque between transmission components. For example, hydraulically actuated friction clutches are used to hold and release elements of a planetary gear set in order to produce the various gear ratios required for the operation of the motor vehicle. A hydraulically actuated clutch typically includes a hydraulic piston housing, a hydraulically actuated piston disposed within the hydraulic piston housing for selectively engaging a multi-plate clutch pack, and a return spring pack assembly for urging the hydraulically actuated piston apart from the multi-plate clutch pack.
A typical return spring pack assembly includes a plurality of coiled springs retained between the two co-axially located spring retainer plates. The coiled springs are evenly angularly spaced on the spring retainer plates. Each of the coiled springs includes a length relatively larger than the diameter of spring. During high speed rotation of the return spring pack assembly about a rotational axis, the centrifugal forces distort and deform the coiled spring by bowing the length of the coiled spring radially outward with respect to the rotational axis.
The deformation of the plurality of springs bowing radially outward causes compression instability which may result in a variable spring constant. The variable spring constant may produce a variable and unpredictable force in the opposition to the hydraulic force that actuates the piston to engage the multi-plate clutch pack. In addition, the instability deformation of the springs may clause excessive wear of the multi-plate clutch pack because the clutch plates are not fully engaged resulting in slipping of the clutch plates, and possible instability of the overall clutch assembly.
Thus, there is a need for an improved return spring pack assembly that reduces or eliminate the deformation of the coil spring due to centrifugal forces.